This invention relates to an improved optical lithography method and apparatus and, more particularly, to an optical lithography method and apparatus having a pulsed laser light source and stimulated raman shifting to produce single or multiple wavelength exposure of a photosensitive element at high resolution.
The drive toward higher density circuitry in microelectronic devices has promoted interest in a variety of high resolution lithographic techniques which require the ability to produce finer resolution patterns at high production rates. In optical lithography, the improvement in resolution that results by use of shorter wavelengths is well known. As a result of these considerations, an effort has been made to develop processes and materials which require exposure in the deep UV spectral region. The light source traditionally used in these prior art systems has been either a deuterium lamp or a xenon-mercury arc lamp. The problem with using such lamps is that insufficient power is available from them in the desired spectral region. For a typical lamp in a typical system, the total deep UV power that can be collected for use is in the few tens of milliwatts range, so that the exposure time for resists that are sensitive in the deep UV are typically several minutes.
It is known that lasers generally produce an intense output, and lasers are known in the prior art which produce outputs in the deep UV spectral region. However, the use of lasers in projection photolithography has traditionally been considered unattractive due to the spatial and temporal coherence characteristics of most lasers. When a field of dimension larger than a few wavelengths is illuminated by a Gaussian beam through some optical components, different parts of the original wavefront interfere constructively and destructively at the sample surface due to imperfections in the various optical surfaces at which the laser beam is reflected or refracted. This interference produces a random pattern called speckle. The presence of speckle completely rules out image formation with feature sizes at the micrometer or submicrometer level, so this characteristic eliminates the use of lasers in projection systems. For this reason, the use of lasers in prior art pattern writing has been in the scanning spot mode, where the beam is focused on the sample and suitably deflected to directly write the desired pattern on the substrate. It will be recognized that this mode of operation requires a CW (continuous wave) laser output and additional high precision deflection mechanisms, and the resultant exposure times are still much longer than desirable.
Another problem that was encountered in prior art photolithography systems is the fact that many otherwise good photoresist materials have been considered undesirable simply because the peak of their spectral performance does not match in wavelength with the emission lines available from the various conventional lamps. As a result of this problem, a considerable amount of development work has been, and continues to be, conducted in an attempt to shift the peak sensitivity region for the photoresist materials to a wavelength at which a suitable emission line is available from the conventional lamp sources.
A further problem that was encountered in the prior art relates to the standing waves produced due to interference between the incident radiation and that reflected from the substrate and from the photosensitive medium-air interface. These standing waves reduce the edge definition of the pattern and limit the effective resolution.